In the metal-processing industry, the method of producing phosphate coatings by means of aqueous zinc phosphate solutions is employed on a large scale. The phosphate layers produced by means of this method on the treated metal surfaces are particularly useful to facilitate sliding, as a preparation for the chipless cold working and for protection against corrosion and as a surface for applying lacquer.
Such phosphatizing solutions usually have a pH-value between 1.8 and 3.8 and contain zinc and phosphate ions as main components. In addition to the cation zinc further cations may be present, e.g. ammonium, calcium, cobalt, iron, potassium, copper, sodium, magnesium, manganese. To accelerate the formation of the phosphate layer, oxidants such as bromate, chlorate, nitrate, nitrite, organic nitro compounds, perborate, persulfate or hydrogen peroxide are generally added to the phosphatizing solutions. To optimize the layer formation on certain materials, there is for instance added fluoride, silicofluoride, boron fluoride, citrate and tartrate. Due to the large number of individual components and their possible combinations there is obtained a plurality of different compositions of the phosphatizing solutions.
A special type of phosphatizing method is represented by what is called the low-zinc methods. The phosphatizing solutions used here contain zinc in concentrations of only about 0.4 to 1.7 g/l and in particular on steel produce phosphate layers with a high content of phosphophyllite, which provides for a better lacquer adhesion and a higher resistance to sub-surface corrosion of the lacquer than is commonly achieved through formation of phosphate layers on the basis of hopeite from phosphatizing solutions with a higher zinc content (DEA-22 32 067, EP-A-15 021, EP-A-39 093, EP-A-56 881, EP-A-64 790, K. Wittel: "Moderne Zinkphosphatierverfahren-Niedrig-Zink-Technik", Industrie-Lackierbetrieb, 5/83, p. 169 and 6/83, p. 210).
A comparatively novel development are the phosphatizing methods which among experts are referred to as trication methods. These are low-zinc phosphatizing methods, where by using nickel in amounts of e.g. 0.3-2.0 g/l and manganese in amounts of e.g. 0.5-1.5 g/l phosphate coatings are obtained which are characterized by an increased alkali resistance and are thus important for cathodic electro-dipcoating, in particular of car bodies.
Especially for phosphatizing galvanized or hot-dip galvanized steel strip, methods have been developed which allow the formation of a phosphate layer corresponding to the trication method within a contact time of 3-8 sec. (EP-A-111 246).
The above-mentioned phosphatizing methods have in common that the phosphatizing solution is brought in contact with the workpiece surfaces to be treated by dipping, flow coating or spraying. After the chemical reaction and upon formation of the firmly intergrown crystalline phosphate layer, the removal of phosphatizing chemicals remaining on the surface requires a rinsing treatment, which is usually performed in several stages. As a result, rinsing solutions are produced, which cannot be disposed of in this form, but must rather be supplied to a liquid-waste disposal system.
Although various suggestions were made for reducing or totally eliminating the amounts of rinsing water, rinsing in what is called a rinsing water cascade for instance involves a considerable reduction of the rinsing water produced. A processing of the rinsing waters even produced in a reduced quantity is, however, inevitable. To avoid rinsing waters it has been proposed to employ a zinc phosphatizing method, whose phosphatizing solutions are composed such that virtually all components can be precipitated by means of calcium hydroxide. In this way, the processing of the rinsing water is facilitated considerably, and at the same time this method has the advantage that water of sufficient quality can be recovered for the process. (DE-C-23 27 304). However, such a process has the disadvantage that due to the request for a precipitability of the constituents of the phosphatizing solution the freedom for the adaptation of the composition of the phosphatizing solution to practical requirements is greatly restricted. Finally, methods of producing a conversion coating are known, where after a possibly necessary cleaning and rinsing with water coating solutions are applied and subsequently dried-on. The application of the treatment solution can be effected by dipping or spraying with subsequent squeezing off the excess solution or by means of roll coating, where only the required amount of liquid is applied onto the metal surface. The process of drying on, which is performed subsequent to the application of the treatment liquid, can in principle already be effected at room temperature. In general, it is, however, common practice to employ higher temperatures, where preferably temperatures between 50 and 100.degree. C. are chosen. Such method designed for the preparation of metal surfaces for the subsequent coating with organic layers consists in wetting the metal surface with a phosphatizing liquid that has a pH-value of 1.5 to 3, is free from chromium and in addition to metal phosphate contains soluble molybdate, tungstate, vanadate, niobate and/or tantalate ions (EP-B-15 020). The cationic component of the metal phosphate in solution may be formed by calcium, magnesium, barium, aluminum, zinc, cadmium, iron, nickel, cobalt and/or manganese.
One disadvantage of the last-mentioned method is that due to the required additions of molybdate, tungstate, vanadate, niobate and tantalate ions the method is more expensive than the conventional phosphatizing methods, and another disadvantage is that the phosphate coatings obtained do not satisfy all the requirements existing today, e.g. as regards the alkali resistance and thus resistance in a subsequent cathodic electro-dipcoating as well as the desired corrosion resistance, in particular in conjunction with a subsequent lacquer coating.